The difference between a dry-type transformer and an oil-immersed transformer comes down to one thing: what surrounds the core and windings. A dry-type unit relies on air and solid insulation; an oil-immersed unit submerges the active part in oil that also carries heat away. Fire behaviour, installation rules, overload tolerance, maintenance and cost all follow from that choice.
Most comparisons online are written for 10 kV and 35 kV distribution transformers rated in the hundreds of kVA. If you are specifying 100 VA to 10 kVA for equipment or a control panel, that advice only half applies. Here is the engineering comparison, then a ten-minute decision sequence for low-voltage buyers.
A dry-type transformer uses air as the cooling medium and solid insulation — impregnated paper, varnish, cast epoxy resin or encapsulated windings — to isolate the turns. Cooling is described as AN (natural air) or AF (forced air with fans). There is no liquid to leak, test or replace.
An oil-immersed transformer places the core and coils inside a steel tank filled with mineral oil or a natural ester. The oil insulates the windings and transfers heat to the tank walls and radiators. Cooling classes run from ONAN through ONAF to OFAF, and the tank brings extra hardware: conservator, breather, oil level gauge and pressure relief device.
If the unit goes inside a building where people work, dry-type is usually the default. If it goes outdoors in a fenced compound and the budget is tight, oil-immersed is usually cheaper per kVA. Those two sentences cover most projects; the rest of this article covers the cases where the choice is close.
Air is a weaker dielectric than oil and a poorer heat carrier. A dry-type unit therefore runs at a higher temperature rise and needs wider ducts and a larger footprint. Dry-type designs are commonly built to 80 K rise on Class F insulation or 100–125 K on Class H; oil-immersed units are usually specified at 55–60 K top-oil rise. Cooling class is printed on the rating plate — an AF-rated unit will not deliver nameplate kVA with the fans off.
Cast-resin dry-type transformers are self-extinguishing and produce limited smoke; open ventilated types are flammable but carry far less fuel than a tank of oil. Oil-immersed units contain a combustible liquid, so most codes require a fire-rated vault, oil containment, suppression or a less-flammable ester liquid before the unit may go indoors. This is the largest hidden cost driver: the transformer is cheaper, the room around it is not.
Oil has high thermal mass and circulates by convection, so an oil-immersed unit tolerates short overloads and high ambient temperatures more gracefully. A dry-type unit heats and cools faster: it responds quickly to load changes but reaches its hot-spot limit sooner in a sustained overload. If the load includes frequent starting currents, size for the peak and review the failure modes in our guide to transformer overheating.
A dry-type unit needs periodic duct cleaning plus torque and insulation checks. Dust, humidity and conductive contamination are its real enemies — in a foundry, a clogged duct is a slow-motion failure. An oil-immersed unit needs oil level checks, breather silica gel replacement, gasket inspection and periodic oil testing for dielectric strength, moisture and dissolved gas.
At the same rating and efficiency class, both designs land within a fraction of a percent on full-load losses; the difference appears at part load. Noise is the other trade-off: a steel tank muffles core hum, so an open dry-type unit in a plant room is usually louder.
Above roughly 100 kVA an oil-immersed unit is usually cheaper to buy. Below that the gap narrows and sometimes reverses, because tank fabrication and oil filling do not scale down well. Compare installed cost: the oil unit may need a fire-rated enclosure, bunding and a maintenance contract; the dry unit may need a ventilated room.
| Factor | Dry-type | Oil-immersed |
|---|---|---|
| Insulation and cooling | Air plus solid insulation; AN or AF | Oil plus solid insulation; ONAN, ONAF, OFAF |
| Fire risk | Low; cast resin self-extinguishing | Combustible liquid; vault or suppression indoors |
| Indoor installation | Normal choice, close to the load | Restricted by code unless ester-filled |
| Short-time overload | Moderate; fast hot-spot rise | High; large thermal mass |
| Maintenance | Cleaning, torque and insulation checks | Oil tests, breather, gaskets, leak repair |
| Typical service life | 20–25 years | 25–30 years with a maintenance programme |
| First cost per kVA | Higher above ~100 kVA | Lower above ~100 kVA |
Search this topic and you land on cast-resin SC(B) and S11 distribution transformers rated 30 kVA to 2,500 kVA at 10 kV or 35 kV. Useful for a substation, misleading for a 1 kVA isolating unit in a control cabinet.
At low voltage the landscape changes in three ways. Oil-immersed designs are rare below about 10 kVA, because tank, oil volume and sealing hardware cannot be justified. The meaningful sub-choice inside “dry-type” is construction, not medium: open ventilated, encapsulated in cast resin, or fully sealed. And the governing standard shifts from IEC 60076-11 towards IEC 61558 for small isolating transformers, which changes the temperature limits and tests you should ask for.
So the real question at low voltage is rarely “dry or oil”. It is which dry-type construction, at what insulation class and IP rating. Our overview of how many types of transformer are there puts this comparison in context.
| Your situation | Recommended choice |
|---|---|
| Indoor plant room, hospital, school, data hall | Cast-resin or encapsulated dry-type |
| Basement or space-limited electrical room | Dry-type, compact frame, AF cooling |
| Outdoor pad or fenced compound, cost-driven | Oil-immersed, ONAN |
| Dusty, humid or washdown environment | Sealed or high-IP dry-type, or oil-immersed outdoors |
| Motor starting and cyclic impact loads | Oil-immersed, or dry-type oversized 25–40% |
| No maintenance team on site | Dry-type |
| Bench or test-bay use with adjustable output | Dry-type variable transformer |
Two genuine advantages survive below the distribution scale. The first is harsh-environment durability: a sealed tank tolerates dust, humidity, salt air and wide temperature swings that would choke an air-cooled winding, which is why pad-mounted units dominate coastal distribution. The second is short-time overload for motor and welding duty, where the oil’s thermal buffer absorbs inrush events that would push a dry-type hot-spot towards its limit.
Everything else at this scale is dry-type territory. A 0.5–5 kVA variable transformer for a test bench is air-cooled by necessity: the brush track and exposed winding could not work immersed, and the application needs adjustable output rather than a fixed ratio. Units such as the TDGC2J series 0.5 kVA to 5 kVA variable transformer show how much of the low-voltage market is settled by construction rather than by cooling medium.
An RFQ that leaves these fields open returns quotes you cannot compare:
Cooling class sets the usable continuous rating, and ambient temperature quietly derates any unit in a hot plant room. Get those wrong and the transformer is correct on paper but undersized in service. The physical differences behind them are covered in our note on transformer construction and iron core structure.
Buying on unit price. The cheaper tank often arrives with a fire-rated room and a bund attached.
Assuming dry-type means maintenance-free. It means no oil programme. Duct cleaning still matters; a clogged air path is a fire risk.
Ignoring the fans. An AF-rated unit with failed fans is an overloaded unit. Specify fan failure alarming or size for AN.
Choose dry-type when the transformer lives indoors, near people or near the load, and when you want a short maintenance list. Choose oil-immersed when the unit lives outdoors, the load is harsh and cyclic, and the site can support an oil inspection programme. At 100 VA to 10 kVA the decision is usually made for you by location; the engineering effort belongs in cooling class, insulation class and IP rating.
If you are narrowing down a specific rating, the step-up and step-down transformer range covers dry-type units from 100 VA to 10 kVA in single-phase and three-phase versions, and our team can confirm the cooling class and derating for your ambient conditions before you commit to a frame size.
Only where the local code allows it. Most codes require a fire-rated vault, oil containment, suppression or an ester liquid. For a normal indoor electrical room, a dry-type unit avoids the question.
Above roughly 100 kVA the dry-type unit usually costs more to buy; below that the gap narrows. On installed cost the comparison often flips, because the oil unit needs fire separation, bunding and an oil programme.
Oil-immersed. The oil’s thermal mass absorbs short overloads and high ambient temperatures better. Dry-type units heat quickly, so size them for the actual peak.
Yes, unless sealed. Air-cooled units need clearance for convection and a supply of clean, cool air. Poor ventilation derates the unit and shortens insulation life.
With proper maintenance, an oil-immersed transformer typically reaches 25–30 years and a dry-type 20–25. In practice, installation environment and loading matter more than the nominal figures.
